1. Field of the Invention
The present invention relates to multiple input multiple output (MIMO) technology, and more particularly, to a link adaptation method suitable for a MIMO system having a multi-antenna structure.
2. Description of the Related Art
In accordance with the proliferation and development of digital devices, digital technology has demanded a high-speed wireless local area network (LAN) system that will operate at data rates of 100 Mbits/sec or higher. To meet such demand, multiple input multiple output (MIMO) technology has been introduced as a candidate for one of the most promising technologies for speeding up next generation wireless LAN systems.
The MIMO technology is classified into a spatial multiplexing technique, which enables higher-speed data transmission by simultaneously transmitting different types of data using multiple transmitting and receiving antennas without the necessity of increasing the bandwidth of an entire system, and a spatial diversity technique, which enables transmission diversity by transmitting one kind of data using multiple transmitting antennas.
Specifically, the spatial multiplexing technique is an adaptive array antenna technique which electrically controls directionality using multiple antennas, in which a plurality of independent transmission paths are established by decreasing the directionality in a narrow-beam pattern, thereby increasing the transmission speed according to the number of antennas. In this case, the same frequency and transmission timing are utilized by the respective antennas.
In a conventional single input single output (SISO)-based wireless LAN system (IEEE 802.11 or 802.11a), a link adaptation method, which varies data transmission methods adaptively to the communication network environment between stations, employs a state of a wireless channel as a factor that can be used in data transmission by a current transmitter, thereby achieving efficient data transmission between the stations.
FIG. 1 illustrates the relationship between a media access control (MAC) layer 20 and a physical layer 10 according to the IEEE 802.11 standard. Referring to FIG. 1, the MAC layer 20 performs data communication with higher layers via a MAC service access point (SAP) 30 and with the physical layer 10 via a physical SAP 40. The physical layer 10 comprises two sublayers, including a physical layer convergence procedure (PLCP) sublayer 11 and a physical medium dependent (PMD) sublayer 12. The PLCP sublayer 11 and the PMD sublayer 12 perform data communication via a PMD SAP 50.
The PLCP sublayer 11 is a layer defined to allow the MAC layer 20 to be minimally associated with the PMD sublayer 12. In other words, the PLCP sublayer 11 converts a service occurring in the MAC layer 20 into a service compatible with an orthogonal frequency division multiplexing (OFDM) physical layer or converts a signal obtained from the OFDM physical layer into a signal compatible with the service occurring in the MAC layer 20 so that the MAC layer 20 can operate independently of the OFDM physical layer.
The PMD sublayer 12 provides the OFDM physical layer with a predetermined signal transmission/reception method. In other words, the PMD sublayer 12, which is closely related to the OFDM physical layer, converts the service occurring in the MAC layer 20 into a service compatible with the OFDM physical layer.
The physical layer 10 of a receiving station, specifically the PLCP sublayer 11, transmits RXVECTOR 60 to the MAC layer 20 via the physical SAP 40. Here, RXVECTOR 60 includes many parameters, including a received signal strength indicator (RSSI). The MAC layer 20 of a transmitting station transmits TXVECTOR 70 to the PLCP sublayer 11 via the physical SAP 40. Here, TXVECTOR 70 includes parameters, such as data transmission rate, power and the like.
FIG. 2A illustrates a function, to which TXVECTOR 70 is applied, and parameters of the function are also shown in FIG. 2A. FIG. 2B illustrates a function, to which RXVECTOR 60 is applied. FIG. 2B also displays parameters of the function. Referring to FIGS. 2A and 2B, TXVECTOR 70 is used as a factor of a function PHY-TXSTART.request, and RXVECTOR 60 is used as a factor of a function PHY-RXSTART.indicate.
More specifically, as shown in FIG. 2A, the TXVECTOR 70 includes parameters LENGTH, DATARATE, SERVICE, and TXPWR LEVEL. The parameter LENGTH indicates the number of data octets to be transmitted from a MAC layer of a transmitting station to a receiving station via a physical layer of the transmitting station and has a value between 1 and 4095. The parameter DATARATE indicates a transmission rate of signals transmitted over a wireless LAN, which can be selected among transmission rates supported by the IEEE 802.11a standard, i.e., 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Among the transmission rates, 6, 12, and 24 Mbps are essentially supported. The parameter SERVICE includes 7 null bits reserved for initialization of a scrambler and 9 null bits reserved for later use. The parameter TXPWR_LEVEL is used for determining the power of signals to be transmitted and has a value between 1 and 8.
As shown in FIG. 2B, the RXVECTOR 60 includes parameters LENGTH, RSSI, DATARATE, and SERVICE. The parameter LENGTH indicates a value of a length field of a received PLCP header and has a value between 1 and 4095, similar to LENGTH of the TXVECTOR 70. The parameter RSSI indicates the energy or intensity of a signal detected from an antenna of a receiving station that is currently receiving data from a transmitting station and is determined when receiving a PLCP preamble. The parameter DATARATE indicates a transmission rate of the data currently being received by the receiving station. Similar to DATARATE of the TXVECTOR 70, DATARATE of the RXVECTOR 60, may be set to one of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The parameter SERVICE of the RXVECTOR 60 includes null bits, analogous to SERVICE of the TXVECTOR 70.
In such a wireless LAN environment, a transmitting station transmits data to a receiving station at a transmission rate. Alternatively, a transmitting station transmits data to a receiving station based on the power of the signal selected by a transmission rate switching mechanism. In this case, the transmitting station performs rate switching through various indicators of states of channels, such as the transmission success proportion of previous frames.
There is another conventional link adaptation method that increases, decreases, or maintains a transmission rate based on a result obtained by comparing an RSSI value measured at an antenna of a conventional SISO system with a predetermined threshold value.
There is a still another conventional link adaptation method that checks packet error rate (PER), which is another parameter used in a link adaptation process, i.e., that checks the transmission success proportion of an acknowledgement (ACK) frame transmitted from a receiving station in response to the transmission of data to the receiving station.
The conventional link adaptation methods described above are inappropriate for MIMO systems using a multi-antenna structure even though they are still effective for SISO systems using a single antenna structure. Therefore, there exists a need for development of a link adaptation method for MIMO systems.